Various compounds have been studied to use for enhancing sensitivity of silver halide photographic materials. It is well known to add amino compounds such as alkylamines, ammonia, hydroxylamines, pyridines, ethylenediamines, imidazoles, etc., to silver halide light-sensitive materials or to developers to accelerate development and obtain high sensitivity. This technique is described in detail, e.g., in Research Disclosure, RD No. 17643 (December 1978), The Theory of the Photographic Process, p. 424 (edited by T. H. James and published by MacMillan Co.), etc.
However, such amino compounds have the defect that, although they accelerate development to some extent to enhance sensitivity, there results considerably increased fog together with an increase in sensitivity when they are added to a silver halide emulsion or to a developer.